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Author Topic: How to wire steppers???  (Read 15736 times)

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Offline jimpinder

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How to wire steppers???
« on: April 03, 2008, 02:53:16 AM »
I'm trying to speed up my axis a bit - well a lot actually, since I cannot move them above 4in per minute at the moment.

Up til now, I thought it might be a mechanical problem with the lathe, but I took the belts off the motors and tried them on their own. I can't get them to spin any faster on their own.

I am running 1/8 step, 1.8 degree, 3 to 1 reduction, 1/10" leadscrew which, I think gives me 48,000 pulses per inch and therefore 192,000 pulses per minute for travel.

The motors are Arc Euro Trade Hybrid Steppers - 220Ncm, 2.5amp 7.5volt - and all in all I am satisfied with them - they are certainly powerful enough to do the job and at £22.95 each are relatively cheap.

They are 8 wire motors - and I have wired them in series.

The $64,000 question is - will they spin any faster if I wire them in parrallel - or is my computer - a Toshiba laptop - just up to its limit (says he looking longingly at the USB driver board just announced).

Anybody any ideas???
Not me driving the engine - I'm better looking.

Offline Hood

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Re: How to wire steppers???
« Reply #1 on: April 03, 2008, 04:42:49 AM »
Jim putting them parallel will give you more torque but I would think lower speed.
 Just out of interest are you using the Chinese drives that Arc Euro and others sell?
 I have just built a small coil winder for a customer and I am having some issues, not sure exactly where the problem is lying yet but I am starting to think harware side of things.

Hood

Offline jimpinder

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Re: How to wire steppers???
« Reply #2 on: April 03, 2008, 06:00:14 AM »
The driver cards are marketed by Routout CNC shop in Wales - don't know where they are made - see Ebay.

Are the Welsh related to the Chinese???

Yes - that is what I though about the steppers - in parallel they will draw twice the current, so should give more torque - but I couldn't see an increase in speed.

I would try it, but the drives are rated at 3 amps max, and the motors are 2.5 amps per segment (7.5v). I am running on 24 volts and I am wondering if they will draw too much current and blow the drive
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Re: How to wire steppers???
« Reply #3 on: April 03, 2008, 06:29:34 AM »
Jim,

Speed of a drive depends on how much torque the stepper can develop and the frictional drag of slideways and the leadscrew, hence the use of ballscrews and linear bearings. The drive system needs to be able to cope with what the stepper requires in terms of amps and volts. As Hood said parallel connection will get you more grunt at low speed at the expense of top speed.
At 4 ipm, 0.1 ins leadscrew and 3:1 reduction, your motor is only managing 120 rpm.  Your top speed for the stepper might be about 800 rpm with a standard driver. You should be able to drive your leadscrew with at 1:1 with this size of stepper motor, so, with luck, you should be able to get up to about 80 ipm! The computer is probably not the limiting factor. Can you run Mach at a higher kernel speed, say 45kHz? Just keep pushing for higher top speed until the motor starts missing steps, then back off a bit.
Have a look at my post :-

http://www.machsupport.com/forum/index.php/topic,4764.0.html

I was trying to improve the speed of the G00 move. The Gecko's start out at 1/10 step at zero revs and "morph" to a simulated full step at a motor speed of 200-300 rpm. Micro stepping is a big help at getting the steppers through their resonant speed range, but not good for ultimate top speed.

Hope this is of some help,

Ian

Offline stirling

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Re: How to wire steppers???
« Reply #4 on: April 03, 2008, 08:34:22 AM »
Hi Jim

I am running 1/8 step, 1.8 degree, 3 to 1 reduction, 1/10" leadscrew which, I think gives me 48,000 pulses per inch and therefore 192,000 pulses per minute for travel.

Absolutely right - or - to put it another way a theoretical resolution per pulse of 0.00002 of an inch. Or more correctly taking out the microstep factor of 8, a theoretical resolution of 0.00016 of an inch. (microsteps should not be included in resolution calculations because they can't be relied upon to be equal in size)

So the first question is - do you require this theoretical resolution? The route from here on in is dependant on that answer. But for the time being let's just look at the status quo.

As you say, at 4 in/min you have a pulse rate of 192,000 ppm or 3200Hz. Clearly Mach/parallel port/USB etc. is not of issue here so let's look at your drivers and motors. Motors first.

The motors are Arc Euro Trade Hybrid Steppers - 220Ncm, 2.5amp 7.5volt - and all in all I am satisfied with them - they are certainly powerful enough to do the job and at £22.95 each are relatively cheap.

They are 8 wire motors - and I have wired them in series.

We need to be clear here exactly how you have them wired and exactly what each coils resistance is.

If indeed you have them wired in series and the 2.5Amps you quote is per coil then you may well be halfing your potentially useful current. But we need to be sure we're both on the same page here if we don't want to damage your drivers.

You need to get your multi-meter out and measure the resistance of 1 of the 4 coils. If you do that and get back we can continue.

Cheers

Ian

Offline jimpinder

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Re: How to wire steppers???
« Reply #5 on: April 03, 2008, 04:06:40 PM »
Thanks to the two Ians and Hood -

Yes - Fired up from the replies I went in the workshop and had a fiddle. I tried altering the incremental steps. I seemed to get more speed from 1/2 step. Full step was a bit too rough. I upped the speed to 10 ins per min. I got the crossslide travelling at that speed but the leadscrew baulked at it.

I get the idea that there is a bit too much resistance for it to accelerate up to top speed. I've tried to adjust the acceleration with some success, but not reliable. I think as Hood said, I could do with a bit more torque to get over the sticky patch.

My motors are wired in BiPolar series, i.e. coil A to coil C  (A - A' - C - C')  and B to D.  The coil resistance is (across two coils in series) 5.8 ohms, across one coil is 3 ohms. This relates to the information about the motors - 2.5amps at 7.5 volts - which is on spec exactly.

My problem is, of course - if I now wire them in parrallel the net resistance will drop to 1.5ohms. I don't know enough about the makeup of stepper motors to know - if I run them at 24 volts how much current will they draw - my drives are RoutOut CNC and in the data sheet are rated at 2.5amps.


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Offline jimpinder

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Re: How to wire steppers???
« Reply #6 on: April 04, 2008, 04:44:59 AM »
Having a devil may care attitude to money - I am going to wire up the leadscrew motor in parrallel and see if the driver will run it without  having a terminal thrombosis.

On a serious note - I put my meter in line with the motor card supply and then jogged up and down. I appreciate it might draw a bit more when working, but jogging (wired in series) the motors only draw 0.5 amps. Even if that went up to 1.5 amps under load, I will still be under my driver capability, even wired in parrallel.

I'm going to try it and see what happens.

The little Chinese man in Wales might get some more trade.
Not me driving the engine - I'm better looking.

Offline stirling

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Re: How to wire steppers???
« Reply #7 on: April 04, 2008, 04:48:37 AM »
Jim - don't do this - you'll likely blow your driver. I'm currently composing a solution for you.
Your meter is not telling you the truth.
« Last Edit: April 04, 2008, 04:51:05 AM by stirling »

Offline stirling

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Re: How to wire steppers???
« Reply #8 on: April 04, 2008, 05:14:58 AM »
Sorry Jim - had to be quick off the mark there - don't want to see you trashing things unnecessarilly  ;D

I think as Hood said, I could do with a bit more torque to get over the sticky patch.

My motors are wired in BiPolar series, i.e. coil A to coil C (A - A' - C - C') and B to D. The coil resistance is (across two coils in series) 5.8 ohms, across one coil is 3 ohms. This relates to the information about the motors - 2.5amps at 7.5 volts - which is on spec exactly.

My problem is, of course - if I now wire them in parrallel the net resistance will drop to 1.5ohms.

This is good news. Now to get that torque. Wire your motors in "bipolar single coil" configuration.
Basically this means you only wire 1 coil of each phase and forget the other two coils. Doing this you'll have a phase resistance of your 3 ohms (actually it should theoretically be 2.8 but it's near enough)

You'll effectively double your phase current and yet still be (just) inside or at your limit (of the driver) of 2.5Amps at around 2.3 to 2.5 Amps. (actually you should be fine because the manufacturers allways give a little leeway).

You should now see quite an improvement but next we'll look at how to use that extra torque to get even more speed.

BTW the reason your meter is not giving you the true story is that if you're measuring the current "drawn" by the drivers from the PS then remember what I said in the other thread about chopper drivers drawing less current than they output. Well this is part of it. But also, the current is being drawn in high frequency pulses and no "regular" meter can read this oscilating current fast enough so it gives you a much lower reading. BUT the real killer is that it's the power transistors that are delivering the FULL motor current that will be maxed out.

Cheers

Ian




« Last Edit: April 04, 2008, 05:24:56 AM by stirling »
Re: How to wire steppers???
« Reply #9 on: April 04, 2008, 05:24:11 AM »
Jim,

The other Ian is correct - DON'T TRY IT.
Resistance is not a good measure in this case, inductance is the important parameter. Measuring current with a meter, only gives you the average current draw, the waveform is a pulsed, square wave and as far as your drives are concerned, it is the peak current that will blow them. I have Routout drives on my Novamill and will post later giving Mach max velocity and acceleration.

Ian